The Dryad is one of those straight-out-of-mythology creatures that doesn't –in my experience– see a lot of table time. Like the unicorn, I think its lack of hostility makes it an uncommon "monster." It's sort of a shame, because what it lacks as a combat opponent for the PCs, it makes up for as an interesting –and slightly spooky– wilderness encounter.
Stat-wise, the dryad isn't particularly scary.
Dryad (from Cook)
Move: 120' (40')
Att: special (charm)
No. Appearing: 0 (1d6)
They are described as "extremely shy and non-violent" (hence the low morale score and lack of combat damage). Their ability to "merge" into their tree makes it unlikely that they would be seen if they didn't wish to be.
One can safely assume a dryad is pretty aware of what goes on in her woods (at least the immediate area). She can't go far from her tree, so perhaps woodland creatures bring her news. Her symbiotic dependence on her tree makes her extremely vulnerable, too. A group of PCs wandering the forest, perhaps seeking the location of a dungeon or similar could do worse than to question a dryad (assuming they locate one).
Her charm person ability is quite powerful, forcing a save at a -2 penalty. The really alarming thing about it isn't falling under the dryad's glamer, it's the next part:
"A charmed character will approach the [dryad's] tree and be drawn into it. Unless rescued immediately, the victim will never be seen again." (emphasis mine)Eek.
So a simple charm person turns into effectively a save or die situation for the PC, at least as far as playing that character again is concerned. What's more, any good loot he might be carrying wanders into the lumber too!
All this means that PCs dealing with a dryad will want to stay on friendly –but not too friendly– terms. After all, that high CHA character might just catch her eye.